This invention relates to a mechanism for changing the drive ratio between a drive mechanism's input and output drives. More specifically, the invention relates to motor vehicles and such a mechanism for reducing the speed of engine drive accessories while the engine is accelerating the vehicle.
Vehicles having drive mechanisms for reducing the speed of a plurality of engine-driven accessories, in response to predetermined engine speed thresholds, are well-known. Most of these mechanisms were proposed during large engine eras, were mainly intended to reduce or optimize accessory size and to improve fuel economy during relatively steady-state operation of the vehicle. Consequently, the prior art mechanisms are of little value during urban cycle driving, where engine speeds are relatively low and where about 50% of the driving time is spent accelerating the vehicle. Accordingly, the prior art mechanisms do little to improve urban cycle fuel economy and/or to increase horsepower available for accelerating the vehicle.
Further, many of the prior art mechanisms have characteristics which make them unacceptable. For example, one class of such mechanisms are downsized versions of large, complex ratio change transmissions which, in many cases, are too bulky for the limited installation space available, are unacceptably expensive, and/or unreliable. Another class of such mechanisms, which are basically two clutches disposed in parallel between the engine and accessories, are not adaptable to provide substantial fuel economy improvement during urban cycle driving and/or to increase horsepower available for accelerating a vehicle.
One example of such a two-clutch mechanism is disclosed in U.S. Pat. No. 3,444,748. The mechanism is per se compact in that it requires only enough space for two output pulleys or powerpaths between the drive mechanism and the accessories. One of the pulleys or powerpaths drives the accessories via a viscous coupling at relatively high rotational speeds relative to the engine when engine speeds are low. The other pulley or powerpath drives the accessories via an overrunning clutch at reduced speeds relative to the engine when engine speeds are high. This arrangement, of course, requires two belt pulleys for each accessory and at least two drive belts, or an intermediate three belt pulley which is connected to the drive mechanism via two belts and with the accessories via a third belt. Hence, the added pulleys and drive belt add to the overall cost and space of the total accessory drive package. Further, the mechanism is not readily controllable to optimize or improve urban cycle fuel economy and/or to increase available horsepower for vehicle accelerations, since the mechanism is designed and intended to be responsive only to engine speed. Drive transition from the high-speed pulley to the low or reduced-speed pulley is effected by a pump in the viscous coupling in response to engine speeds. As the engine speed approaches a predetermined threshold speed, the pump becomes active and reduces the amount of viscous fluid in the coupling available for transmitting torque to the high-speed pulley, thereby increasing slip in the coupling due to accessory drive resistance. The slip reduces accessory speed relative to engine speed and reduces the output speed of the overrunning clutch until the clutch locks up due to its engine-driven input speed attempting to overrun the output speed.
Another example of a two-clutch mechanism is the double electromagnetic clutch arrangement disclosed in U.S. Pat. No. 3,429,192. This mechanism is intended as a two-speed drive for a single accessory, such as an air-conditioning compressor. If the mechanism were employed as a drive for a plurality of accessories, it would be somewhat analogous to the mechanism of U.S. Pat. No. 3,444,748 in that it would include two output drive pulleys of different size and two drive belts. Hence, the added pulleys and drive belt would add to the overall cost and space to the total accessory drive package. The electromagnetic clutches of this mechanism appear to be readily controlled to provide improved urban cycle fuel economy and/or to increase horsepower available for accelerating a vehicle. However, shifts from one clutch to the other would have to be performed while substantial speed differences exist between the clutches. Hence, the shifts would impose substantial shock loads on the accessories, the drive belt, the clutch, and the vehicle.